This invention relates to acoustical-electrical systems possessing parameters which permit self-oscillation induced by positive feedback. More particularly, this invention relates to systems in which provision is made to suppress this type of self-oscillation.
Many systems are known for converting voice signals, i.e., those in the audible range, to the electrical equivalent form, amplifying these electrical signals, and using these amplified signals to drive a follow on device, e.g. a loud speaker, a recording device, a telephone transmitter circuit, or the like. In known systems of this type, feedback paths typically exist which adversely affect system performance. For example, in a simple public address system, acoustical feedback paths are normally present which permit a portion of the acoustic signals emanating from the loud speaker to be coupled to the microphone. In a full duplex telephone line repeater system, on the other hand, electrical feedback paths are present which permit a portion of the amplified electrical signals to be electrically fed back to the input portion of the circuit in a closed loop. Moreover, in an amplified telephone system, having a telephone transmitter microphone and a speaker coupled to the telephone lines via a hybrid circuit, both acoustic and electrical feedback paths are present which can succeed in both acoustically and electrically coupling back signals to an input portion of the system.
In systems having either acoustic or electric feedback or a combination of both, it has been found that positive feedback occurs at those frequencies at which the phase shift of the loop is an integral multiple of 360.degree. and at which the net system voltage gain is equal to unity, i.e., those frequencies which satisfy the Bode stability. criteria. When these criteria are met, the result is a ringing or squealing signal which overrides or masks the desired signals.
Efforts to suppress such unwanted feedback, in which an amplified signal re-enters the amplifying channel to cause squeal, have centered about providing conventional frequency domain filters which attempt to eliminate or reduce the amplitude of those frequencies satisfying the Bode criteria. The insertion of a convention notch or band pass frequency filter in the amplifying channel, however, has not been found to be particularly effective since the net effect of this insertion is to increase the system phase shift at the upper band edges of the band pass frequency for which the system is designed. This has the effect of merely lowering the frequency at which positive feedback occurs, without eliminating the feedback.